1. Field of the Invention
The present invention relates to a fuel-sorbing device, such as a canister to be attached to a fuel tank of an automobile. More particularly, it relates to a fuel-sorbing device using a particular sorbent.
2. Description of the Related Art
A relatively large amount of fuel evaporates at the time when it is fed by a feeding gun into a fuel tank of an automobile. Part of the fuel contained in a fuel tank or in the float chamber of a carburetor also evaporates not only when the automobile is running, but also when it is not running. Because of this, a canister filled with a sorbent is positioned between the tank and the open space, in order to capture the evaporated fuel and to avoid its leakage into the atmosphere.
Fuel-sorbing devices charged with a sorbent are also used in order to capture fuels evaporated from fuel-storage tanks other than those of automobiles, as well as to capture fuels leaked therefrom.
Powders or granules of active carbon have been used as a sorbent in the prior fuel-sorbing devices, including canisters.
When a canister has sorbed evaporated fuel almost to its maximum sorbing capacity, air is introduced into the device in order to desorb (or purge) the fuel out of the active carbon. The purged canister is again used to sorb evaporated fuel.
However, when such a canister is used, there often occur cases wherein evaporated fuel is captured only insufficiently, and part of it released into the atmosphere.
That is, the fuel-capturing capability (or working capacity) of active carbon is not sufficiently high.
Of fuel components captured by the active carbon housed in a canister, relatively small molecules containing 5 carbon atoms or less can be readily desorbed during the purging (desorbing) step, whereas it is difficult to completely desorb larger molecules (or molecules containing 6 or more carbon atoms) by the purging.
Because of this, when active carbon is subjected to purging repeatedly, larger molecules gradually accumulate in the active carbon, and the working capacity of the sorbent deteriorates with the lapse of time.
A marked deterioration in the working capacity of carbon atom is also observed when evaporated fuel is condensed and liquefied on the inner wall of a canister or a connecting pipe, and the active carbon is soaked with the liquefied fuel. If active carbon is used in an increased amount sufficient to rectify the above deficiency, the size of the canister would become undesirably large, posing various restrains on the designing of automobiles, or the like.
Thus, prior canisters using active carbon are often insufficient in their capability of capturing evaporated fuel and hence, part of evaporated fuel is released into the atmosphere, causing air pollution.